Overall requirements for the Next Generation (NG) architecture (see TR 23.799, Study on Architecture for Next Generation) and, more specifically the NG Access Technology (see TR 38.913, Study on Scenarios and Requirements for Next Generation Access Technologies) will impact the design of the Active Mode Mobility solutions for the new RAT (NR) (see RP-160671, New SID Proposal: Study on New Radio Access Technology, DoCoMo) compared to the current mobility solution in Long Term Evolution (LTE). Some of these requirements include the need to support network energy efficiency mechanisms, the need to account for future developments, and the need to support a very wide range of frequencies (up to 100 GHz).
In the context of Neighbor-cell Relation (NR) handover, a measurement framework can be used to support dynamic network-triggered Mobility Reference Signal (MRS) measurement and reporting, and/or periodic MRS measurement and event based reporting (as in LTE). MRS may also stand for a Measurement Reference Signal. FIG. 1 shows a source access node 70, a target access node 80 and a user equipment (UE) 50 in the context of downlink measurement based handover. Under normal circumstances, user data and an MRS configuration may be exchanged between the UE 50 and the source access node 70. The MRS configuration messaging in the example of FIG. 1 is performed at the RRC level, or at Level 3 (L3). Upon a beam switch trigger, MRS measurements are made by the UE 50. Optionally, an MRS configuration and an activate measurement command are sent to from the source access node 70 to the UE 50. After measurements of downlink reference signals are reported by the UE 50 to the source access node 70, a handover decision is made by the source access node 70. A handover command can then be sent as a Radio Resource Control (RRC) connection reconfiguration message from the source access node 70. A handover command can also be implicitly indicated by an uplink grant message from the target access node 80.
Upon the reception of an RRC reconfiguration message or the best beam detection, the UE 50 contacts the target access node 80 using an Uplink Synchronization Signal (USS), which may be coupled with the MRS sequence of the target beam/cell/node, so that the selected USS can be used to indicate the detected best beam. The USS also serves as an uplink timing reference since the UE 50 needs uplink time synchronization when changing access nodes. USS has a similar design as Physical Random Access Channel (PRACH) preamble in LTE and is intended for the uplink timing advance (TA) calculation, uplink frequency offset estimation and uplink beam identification. As a response or a subsequent message to the USS, the target access node 80 sends an uplink grant, including the TA to establish uplink synchronization with the target access node 80.
As an alternative to the downlink measurement based handover, there could be uplink measurement based handover relying on the same principles, as shown in FIG. 2. This may involve the source access node 70 optionally sending a USS configuration and a USS activation command to the UE 50. The USS is used to indicate a detected test beam and to serve as an uplink timing reference. The source access node 70 and/or the target access node 80 perform measurements on signals and then determine the best beam. Yet, in this case, the beam switch command is an MRS, transmitted on a time/frequency resource indicated in the USS configuration message.
However, it is recognized herein that the current use of MRS configuration in these instances is not optimal, because the UE 50 may not need to measure/report all the configured MRSs all of the time. Also, the UE 50 may not have a sufficient time budget to be reconfigured at the RRC level for every MRS modification or configuration.